The present invention relates to a power transmission device having a torque limiting function. More particularly, the present invention relates to a power transmission device incorporated into a compressor of an air conditioner for vehicle use.
In a conventional power transmission device for transmitting power to a compressor, in order to avoid the occurrence of a problem in which a belt of the compressor, which is used for transmitting power, is damaged when the compressor has seized due to a heavy load, a torque limiter is arranged in the power transmission device. For example, in the case of a torque limiter shown in the official gazette of JP-A-2003-206950, a portion of the power transmitting passage is composed of a screw-in connection.
In the official gazette of JP-A-2003-206950, as shown in FIG. 22, the adapter B, from which the cylindrical limiter attaching portion B1 integrally protrudes, is press-fitted and fixed to the rotary shaft C. On the inner circumferential face of this limiter attaching portion B1, the female screw B2 is formed. On the other hand, the torque limiter A, which is fixed to the pulley D, is integrally composed of a disk-shaped flange portion A1 and a cylindrical connecting portion A2. On the outer circumferential face of the connecting portion A2, the male screw A3 is formed. When the torque limiter A is engaged with the adapter B by means of screwing in this way, the pulley D is connected to the rotary shaft C via the torque limiter A.
In this torque limiter system, to which the above screwing engagement is applied, by utilizing an excessively high axial force which is generated in the screwing engagement portions A3, B2 by an excessively high intensity of torque generated at the time of seizure of the compressor, a portion of the power transmitting path is broken so that power transmission can be shut off.
However, in this torque limiter system, the fastening is accomplished by means of screwing. Therefore, the operation accuracy of the torque limiter is affected by the intensity of torque generated at the time of seizure of the compressor, and by the screwing portions A3, B2 which are engaged with each other and by the coefficient of friction on the seating face which is a contact face of the torque limiter A with the adapter B. Especially, in the case of the power transmission device shown in the official gazette of JP-A-2003-206950, when the coefficient of friction is changed by the influence of corrosion of the screw engagement portion caused by the water which has entered the device from the outside, the operational torque of the torque limiter changes with time.
Further, in the case where the screwing engagement portion is covered with oil so as to stabilize the coefficient of friction, oil flows out while the device is actually being used, and the coefficient of friction changes, which affects the accuracy of the torque limiter.